The present invention relates to an optical processing apparatus such as, for example, one for forming via holes in a printed circuit board by means of a light beam such as a laser beam through masking.
FIG. 15 illustrates a typical example of such an optical processing apparatus. In this figure, a mask, which is generally designated at reference numeral 1, includes a transparent plate or board is formed of synthesized quartz or the like, and a reflecting or masking portion 1c having a high reflection factor and placed on a surface of the transparent board 1a with a circuit pattern of a predetermined configuration provided thereon through the reflecting portion 1c. The reflecting portion 1c is formed, for example, of a thin aluminum film or the like which is vapor deposited on the surface of the transparent board 1a while leaving a circuit pattern 1b thereon through which a beam of light 3 in the form of a laser beam can pass toward a substrate 5 which is disposed behind the mask 1 at a predetermined distance therefrom. A reflector 2 is disposed at a predetermined distance from the mask 1 in a direction of incident light and in a parallel relation with respect thereto for reflecting light beams reflected from the reflecting film 1c. An optical focusing system 4 such as a convex lens is also disposed at a distance from the mask between the mask 1 and the substrate 5.
In operation, as illustrated in FIG. 15, a portion of a laser beam 3 first irradiated on the mask 1 through an adjacent or upper edge of the reflector 2 passes directly through the non-masked portion 1b of the circuit pattern and the transparent board 1a to be utilized for optically processing the substrate 5 disposed behind the convex lens 4, whereas the remaining portion of the incident laser beam 3 is reflected at the reflecting film 1c on the surface of the transparent board 1a toward the reflector 2. The remaining portion of the laser beam 3 thus reflected from the reflecting film 1c is again reflected at the surface of the reflector 2 toward the mask 1. In this regard, the laser beam 3 is initially incident on the mask 1 at a predetermined angle relative to the normal or perpendicular line with respect to the surface of the mask 1, so that the laser beam 3 once reflected from the surface of the reflector 2 arrives at a second location deviated from a first location on the mask 1 to which the laser beam 3 is initially incident. That is, the second location is deviated from the first location by a certain distance in a downward direction in FIG. 15. Thereafter, such a process is repeated a number of times until the reflected laser beam 3 finally comes to an opposite or lower edge of the mask 1 and escapes outwardly therefrom. During the repeated reflections, the laser beam 3 once having passed through the non-masked or pattern portion 1b in the reflecting or masking film 1c is focused on a surface of the substrate 5 under the action of the convex lens 4, whereby the surface of the substrate 5 is optically processed by the laser beam 3 to provide a circuit pattern thereon which corresponds to the non-masked circuit pattern 1b on the surface of the transparent board 1a. In this manner, a portion of the laser beam 3 incident to the surface of the reflecting film 1c on the transparent board 1a is repeatedly reflected between the parallel-disposed reflecting film 1c and reflector 2 and hence it is repeatedly reused a number of times for enhancing the total efficiency of the laser beam 3. This enables as large an area of the substrate 5 as possible to be processed by a fixed amount of laser beam without increasing the laser power.
With the above-described optical processing apparatus, however, the strength or magnitude of the reflected laser beam 3 repetitively reflected between the reflecting film 1c and the reflector 2 is distributed non-uniformly over the surface of the mask 1, and there is a considerable portion of the reflected laser beam escaping outward from between the mask 1 and the reflector 2, thus resulting in non-uniformity in processing the substrate 5 and in reduction in the efficiency of available laser energy. A more detailed explanation of this will be given below.
FIGS. 16 and 17 illustrate how the laser beam 3 repetitively reflected between the mask 1 and the reflector 2 travels. FIG. 16 is a perspective view showing the state in which the mask 1 is placed on an x-y plane with the x axis oriented in a direction in which the laser beam 3 proceeds. FIG. 17 is a cross sectional view looking at FIG. 16 in the direction of the x axis from the front to the back of the drawing sheet. FIG. 18 illustrates the distribution of the strength or magnitude of the laser beam 3 over a surface of the mask 1 in the x-axis direction. In fact, as the laser beam 3 is travelling while being repetitively reflected between the two parallel reflecting surfaces 1, 2, the strength or magnitude of the laser beam 3 gradually decreases. In particular, this tendency becomes more remarkable as the rate of opening of the mask 1 (i.e., the rate of an opening area of the mask 1, against which the incident laser beam 3 directly impinges, to the total area of the mask 1) or the angle of incidence of the laser beam 3 increases. As a result, as shown in FIG. 18, the strength or magnitude of the reflected laser beam at the surface of the reflecting film 1c decreases as it proceeds from the upper edge to the lower edge of the mask 1, thus resulting in irregular or non-uniform distribution of the energy density. In addition, as can be clearly seen from FIG. 17, the total width (i.e., the length in the y-axis direction perpendicular to the x-axis direction) of the laser beam 3 gradually increases as the laser beam 3 proceeds while being reflected between the mask 1 and the reflector 2, so that a portion of the reflected laser beam 3 finally leaks or escapes outwardly of the mask 1 and can not be reused for optical processing, thus reducing the efficiency or the rate of utilization of the laser beam 3. As a result, the distribution in the strength or magnitude of the laser beam 3 over the surface of the mask 1 in the y-axis direction becomes non-uniform.
Moreover, FIGS. 19 and 20 illustrate the distributions of the strength of the laser beam 3 over the surface of the mask 1 with a relatively large angle of incidence .theta..sub.0 and a relatively small angle of incidence .theta..sub.0, respectively, of the laser beam. As can be seen from these figures, the strength of the laser beam 3 is greater with the small incident angle than with the large incident angle. In this case, however, as illustrated in FIGS. 21 through 23 which illustrate how the laser beam 3 is incident to the mask 1 and initially reflected therefrom, the laser beam 3 has a tendency to spreads or widen as it proceeds, so an increasing portion of the laser light 3, which is once reflected from the mask 1 and proceeding to the reflector 2, escapes or leaks from an edge of the reflector 2 in accordance with the decreasing angle of incidence. In FIG. 21, the incident laser beam and the reflected laser beam are depicted concurrently in a superposed fashion but by different hatching, whereas they are individually depicted in FIGS. 22 and 23, respectively. It is evident from FIG. 23 that a portion of the laser beam 3 initially reflected from the mask 1 leaks from an edge of the reflector 2, thus resulting in an energy loss.